1. Field of the Invention
The present invention relates to a device for driving a piezoelectric element.
2. Description of the Related Art
In a well known device for charging and discharging a piezoelectric element, the electric charge is charged to a power condenser in a piezoelectric element through a charging coil, and the electric charge in the piezoelectric element is discharged through a discharging coil. This device, however, has a problem in that the discharged electric charge is wastefully consumed.
To solve the above problem, in a well known device for charging and discharging a piezoelectric element a part of the discharged electric charge is recovered through a power condenser, to thus lower the power consumption (refer to Japanese Utility Model Publication No. 62-117251). In the above device for charging and discharging a piezoelectric element, first an electric charge in a piezoelectric element is discharged to a high potential side of a power condenser through a discharging coil and a switching means, during a discharging operation, to thus recover the discharged electric charge by the power condenser, and then the electric charge in the piezoelectric element is discharged to the ground through a discharging coil and a switching means when the terminal voltage of the piezoelectric element reaches a predetermined setting voltage or lower.
An amount of expansion of the piezoelectric element is determined by the difference between the terminal voltage (positive voltage) of the electric element during a charging operation and the minimum terminal voltage (negative voltage) of the piezoelectric element after a discharging operation is completed. Since the terminal voltage of the piezoelectric element during the charging operation is influenced by the terminal voltage of the piezoelectric element prior to charging, i.e., the minimum and negative terminal voltage of the piezoelectric element after the charging operation is completed, it is necessary to control the minimum and negative terminal voltage of the piezoelectric element after the completion of the discharging operation to a constant negative voltage, to thereby control the amount of expansion of the piezoelectric element to a constant value. As disclosed in the above-mentioned Japanese Unexamined Utility Model Publication No. 62-117251, however, where the electric charge in the piezoelectric element is discharged to the ground when the terminal voltage of the piezoelectric element reaches a predetermined setting voltage or lower, the terminal voltage of the piezoelectric element is later reduced and becomes a minimum and negative terminal voltage, which is not fixed at that time, and therefore, the amount of expansion of the piezoelectric element cannot be precisely controlled to a constant value.